Replace the monolithic RigNonUniformRefinement class with a proper
polymorphic hierarchy: RigRefinement (abstract base), RigNoRefinement
(identity), RigUniformRefinement (O(1) uniform), and
RigNonUniformRefinement (per-cell custom fractions).
Ownership uses std::unique_ptr<RigRefinement>, consumers receive
const RigRefinement&. This eliminates the misleading class name,
removes effectiveRefinement() indirection, and gives each refinement
mode an appropriately optimized implementation.
